/**
 * Original Filesystem Cache implementation by babel-loader
 * Licensed under the MIT License
 *
 * @see https://github.com/babel/babel-loader/commits/master/src/fs-cache.js
 * @see https://github.com/babel/babel-loader/commits/master/src/cache.js
 */

/**
 * Filesystem Cache
 *
 * Given a file and a transform function, cache the result into files
 * or retrieve the previously cached files if the given file is already known.
 *
 * @see https://github.com/babel/babel-loader/issues/34
 * @see https://github.com/babel/babel-loader/pull/41
 */
import fs from 'fs';
import os from 'os';
import { join } from 'path';
import { promisify } from 'util';
import zlib from 'zlib';
import { createHash } from 'crypto';

import findCacheDir from 'find-cache-dir';

// Lazily instantiated when needed
let defaultCacheDirectory = null;

const readFile = promisify(fs.readFile);
const writeFile = promisify(fs.writeFile);
const gunzip = promisify(zlib.gunzip);
const gzip = promisify(zlib.gzip);

/**
 * Read the contents from the compressed file.
 *
 * @async
 * @params {String} filename
 * @params {Boolean} compress
 */
const read = async (filename, compress) => {
  const data = await readFile(filename + (compress ? '.gz' : ''));
  const content = compress ? await gunzip(data) : data;

  return JSON.parse(content.toString());
};

/**
 * Write contents into a compressed file.
 *
 * @async
 * @params {String} filename
 * @params {Boolean} compress
 * @params {String} result
 */
const write = async (filename, compress, result) => {
  const content = JSON.stringify(result);

  const data = compress ? await gzip(content) : content;
  return writeFile(filename + (compress ? '.gz' : ''), data);
};

/**
 * Build the filename for the cached file
 *
 * @params {String} source  File source code
 * @params {String} identifier
 * @params {Object} options Options used
 *
 * @return {String}
 */
const filename = (source, identifier, options) => {
  const hash = createHash('md4');

  const contents = JSON.stringify({ source, options, identifier });

  hash.update(contents);

  return `${hash.digest('hex')}.json`;
};

/**
 * Handle the cache
 *
 * @params {String} directory
 * @params {Object} params
 */
const handleCache = async (directory, params) => {
  const {
    source,
    options = {},
    transform,
    cacheIdentifier,
    cacheDirectory,
    cacheCompression,
  } = params;

  const file = join(directory, filename(source, cacheIdentifier, options));

  try {
    // No errors mean that the file was previously cached
    // we just need to return it
    return await read(file, cacheCompression);
    // eslint-disable-next-line no-empty
  } catch (err) {}

  const fallback =
    typeof cacheDirectory !== 'string' && directory !== os.tmpdir();

  // Make sure the directory exists.
  try {
    fs.mkdirSync(directory, { recursive: true });
  } catch (err) {
    if (fallback) {
      return handleCache(os.tmpdir(), params);
    }

    throw err;
  }

  // Otherwise just transform the file
  // return it to the user asap and write it in cache
  const result = await transform(source, options);

  try {
    await write(file, cacheCompression, result);
  } catch (err) {
    if (fallback) {
      // Fallback to tmpdir if node_modules folder not writable
      return handleCache(os.tmpdir(), params);
    }

    throw err;
  }

  return result;
};

/**
 * Retrieve file from cache, or create a new one for future reads
 *
 * @async
 * @param  {Object}   params
 * @param  {String}   params.cacheDirectory  Directory to store cached files
 * @param  {String}   params.cacheIdentifier Unique identifier to bust cache
 * @param  {Boolean}  params.cacheCompression
 * @param  {String}   params.source   Original contents of the file to be cached
 * @param  {Object}   params.options  Options to be given to the transform fn
 * @param  {Function} params.transform  Function that will transform the
 *                                      original file and whose result will be
 *                                      cached
 *
 * @example
 *
 *   cache({
 *     cacheDirectory: '.tmp/cache',
 *     cacheIdentifier: 'babel-loader-cachefile',
 *     cacheCompression: true,
 *     source: *source code from file*,
 *     options: {
 *       experimental: true,
 *       runtime: true
 *     },
 *     transform: function(source, options) {
 *       var content = *do what you need with the source*
 *       return content;
 *     }
 *   });
 */

module.exports = async (params) => {
  let directory;

  if (typeof params.cacheDirectory === 'string') {
    directory = params.cacheDirectory;
  } else {
    if (defaultCacheDirectory === null) {
      defaultCacheDirectory =
        findCacheDir({ name: 'eslint-loader' }) || os.tmpdir();
    }

    directory = defaultCacheDirectory;
  }

  return handleCache(directory, params);
};
